Elongation of tensile test specimen 1

[killswitch06]

I am performing a destructive test in which i will be pulling a very long tensile test specimen.

the speicimen is 63" long with a diamter of 1.5", at the midpint of the specimen it will be notch with a diamter of 1 " .

the materail is a AISI 4340 alloy steel with a min yield of 150 KSI.

here is the part that is confusing me... the material sepc says " Elongation in 2" or $D, min = 10% "

im trying to caclualte how much this test specimen will stretch before failue and im unsure how to apply the 10" number from above...


diagram of specimen

 

[killswitch06]

correction to above " Elongation in 2" or 4D, min = 10%

 

[rb1957]

first, why ae you testing such a large specimen ? full size test of the final part ?

2nd, google "mil hdbk 5" or "AR MMPDS" which'll give you the spec for 4340.

looking under "low alloy steels", 150ksi 4340 is expected to elongate 14% (about 1/7 or 9" for your 63" testpiece).

i'd interprete the 10% as saying you should get more than 10% elongation, or 6.3".

i expect you're also doing hardness testing to verify the temper of the steel ?

Quando Omni Flunkus Moritati

 

[rb1957]

btw, stand Well back ... when this thing lets go, it'll go with a Bang ! (try to video the test and upload for us !!)

Quando Omni Flunkus Moritati

 

[killswitch06]

full size test of final part,

now if the diamter of the specimen was uniform throught i would agree that it would strethc roughly 6"

however there is a notch which brings it down to 1" in the center.

so does this mean that the length i should use in teh equation is the length of which the part is at the 1" diamter?

 

[MikeHalloran]

You are not being perfectly clear on this:
Is the 1" diameter a notch, or a necked down section?

If you are trying to measure the elongation, I think you need a necked down section at least 4" long, with gently radiused transitions to the larger diameter.

With a notch, i.e. a v-groove with a sharp bottom corner, you are measuring something other than elongation.



Mike Halloran
Pembroke Pines, FL, USA

 

[rb1957]

the piece will fail at the notch, at a load of (about) 150ksi*Anotch.
the stress on the unnotched part will be 150*Anotch/Aplain ksi,
the strain will be unnotched stress/E (29E3 ksi),
the deflection will be (roughly) unnotched strain*63"

Quando Omni Flunkus Moritati

 

[killswitch06]

the 1" section is just a v notch, i want the part to fail at the notch. I will not be measuring the stretch of the part with any type of device. I just want to know roughly how much the overall lenght of the test peice will increase before the failure occurs at the notch.

 

[redpicker]

rb1957 almost has it right.
The part will fail at a load of Anotch*UTS since the constraint of the notch will prevent the material from yielding. This is worse-case and assuming no notch sensitivity of the material, which for 4340 is a pretty fair assumption.

UTS is unknown. For 4340 at 150KSI min ys, a Y/T ratio of 0.85 is reasonable (perhaps a bit low, but reasonable). But, the actual YS is not 150,000, better figure on 10% greater, so
YS=150000*1.1 = 165000
UTS = 165000/0.85 = 194,100 KSI

Anotch = 0.785 in^2
Aunntoch = 1.767 in^2

Unnothced stress = 194100*.785/1.767 = 86300 PSI

Strain = 86300/29e6 = .003 or 0.3%
63*0.003= 0.189 in <<(this seems low too me, maybe someone should check my math--On second thought, since there won't be any yielding going on, I guess it's correct).

Your specimen will stretch about 3/16" before it breaks. It will break with little or no yeilding, however, and nearly 75 tons of force will be released instantaneously. If the ends are not constrained, they will be shot out of the fixture like a missle. If they are constrained, whatever is constraining them will be hit with 75 tons.

rp

 

[redpicker]

OK, PSI, not KSI.
194,100 PSI for line 6.

rp

 

[rb1957]

agreed ... in round numbers, the gross section stress (1.5" dia) will be something like 44% (= 4/9) UTS ... slightly less to account for Kt ... and the rest follows quite simply.

this sounds like a structural fuse ... designed to fail at a specific load, yes?

Quando Omni Flunkus Moritati

 

[chicopee]

Looking at the diagram, the notch configuration may be problematic as you may get an elongation much less than the calculated one. Obviously the specimen under test is not the typical "dog bone" type normally used during tensile strength-strain studies. I think that you would get better results about the material if you had better transition between the 1-1/2" dia section and the 1" dia section, and have a 1" dia section of a much longer length than the "V" cut.

 

[killswitch06]

this is essentialy a structural fuse. im not measuring anything about the material , i just want it to break

 

[rb1957]

depending on how tightly you are controlling the failure load, this can be quite an exercise.

i might have gone a different route ... make the bar from two pieces with a lap joint in the middle and use the fasteners in shear as the fuse; i think fasteners are more tightly controlled. but this would be a more "brittle" failure ... maybe you're looking for a ductile failure, with lots of stretch before the big bang.

i'd suggest that you add a sleeve around the groove ... something that isn't structural but would retain the two pieces after failure

Quando Omni Flunkus Moritati